In a medium saucepan, combine the milk with cinnamon sticks, lemon zest, and orange zest. Heat over medium heat until it just begins to simmer, then remove from heat and let steep for 15 minutes.

In a mixing bowl, whisk together egg yolks and 100g of sugar until pale and creamy, about 2-3 minutes. Add the cornstarch and whisk until smooth with no lumps remaining.

Remove the cinnamon sticks and citrus zest from the milk. Slowly pour about 1 cup of the warm milk into the egg mixture while whisking constantly to temper the eggs.

Pour the tempered egg mixture back into the saucepan with the remaining milk, whisking continuously. Cook over medium-low heat, stirring constantly with a wooden spoon, until the mixture thickens enough to coat the back of the spoon, about 8-10 minutes. Do not let it boil.

Once thickened, remove from heat and stir in the vanilla extract. Strain the custard through a fine-mesh sieve into a clean bowl to ensure a smooth texture.

Divide the custard evenly among 6 shallow ramekins or traditional cazuelas. Let cool to room temperature, then cover with plastic wrap and refrigerate for at least 4 hours or overnight until fully set.

Just before serving, sprinkle about 1 tablespoon of sugar evenly over the surface of each custard. Using a kitchen torch, caramelize the sugar until golden brown and bubbling. Alternatively, place under a hot broiler for 1-2 minutes, watching carefully.

Let the caramelized sugar harden for 1-2 minutes before serving. The sugar should crack easily when tapped with a spoon.